The standard deviation of a set of numbers is the average difference of all of those numbers from the mean. It gives you a measure of the variation among the numbers with respect to the mean.
1. Find the mean (or average) of the numbers - add them and divide the total by the amount of numbers you have.
2. Subtract the result (the mean) from each number.
3. Square each of the differences in step 2, and add these results together.
4. Divide this total by one less than the amount of numbers. This result is the variance.
5. Take the square root of the result from step 5 - this is the standard deviation.